On a lot of consumer products there's a resettable thermal fuse built into the heating element. Burning down people's houses is bad for business :)

I'll put my hand up for that one.

Old fashioned non-automatic electric kettles had such a device that handily pushed out the kettle lead thus disconnecting the mains. I was saved from some embarrasment and redectoration by this a third of a century ago.

PS: how could you detect the presence of water? Photocell? Ultrasound? Conductivity between two small contacts 20% of the way up the jug?

The old way in industrial automation is a float sensor, which is basically a reed switch with some buoyancy on the end. With substances that are corrosive, or where there's a cross contamination concern, you drop down to load cells or a rangefinder, both of which are more expensive and fussy. :)

At least in my Keurig: there are 2 tanks, 1 external, 1 internal. There is a float in the external tank that detects if there is enough water in the external to fill up the internal. If so, then it brews the internal tank and then fills it up from the external after the coffee is made. If there is not enough water to fill up the tank, it won't start.